This invention relates to the testing of copper or other foils, and in particular the rupture testing of copper foil for laminates and plated through hole (PTH) deposits as used in printed wiring circuit boards.
Mullen testers of various sorts have frequently been employed for rupture testing the bursting strength of copper, aluminum or other foils and films. Various Mullen testers are described in the literature, and in this connection, reference is made to Smith U.S. Pat. No. 2,332,818, Oct. 26, 1943; Hollis U.S. Pat. No. 2,565,371, Aug. 21, 1951; Lovette U.S. Pat. No. 3,160,002, Dec. 8, 1964; and Schlegel U.S. Pat. No. 3,600,940, Aug. 24, 1971.
Mullen tester devices typically clamp a test sample of sheet material, which can be paper, cloth, fiberboard, metal foil, or synthetic material, between flat plates which have circular openings. Fluid under pressure forces a rubber diaphragm to expand through the apertures of these plates, and to exert a constantly increasing pressure against the unsupported area of the sheet material. This pressure is measured by a pressure gauge. When the bursting point of the sheet material sample is exceeded, the material tears, the pressure drops, and the pressure gauge indicates the burst pressure of the material.
Some of these testers also have a dial micrometer with an operating stem or probe which extends axially through the outer circular aperture against the sample. As the pressure is increased, the sample under stress will deform and dome until the burst or rupture point. The dial micrometer provides an indication of the material deformation under the various stressing pressures, and the maximum deformation corresponding to bursting or rupture.
Other rupture testers have also been proposed, e.g., in Getchell U.S. Pat. No. 2,525,345, Oct. 10, 1950; Biondi U.S. Pat. No. 3,736,794, June 5, 1973; Beckstrom U.S. Pat. No. 3,618,372, Nov. 9, 1971; Zacios U.S. Pat. No. 3,548,647, Dec. 22, 1970; and Tasker U.S. Pat. No. 2,748,596, June 5, 1956. These testers usually involve impacting an object onto the sheet or foil sample. The Tasker device carries out testing in a heated or refrigerated atmosphere.
There has recently arisen a need for improved testing techniques, particularly to establish performance requirements for copper cladding laminate foils and for PTH deposit quality in printed wire multilayer circuit boards (PWMLBs). Because of advances in integrated circuit technology and resultant miniaturization of components, quite sophisticated multilayer circuit boards have resulted. Because of the high cost and high performance requirements of these, the performance characteristics of the metal films employed as printed conductors or as plated through holes, should be known rather precisely.
The most recent impetus for performance improvement stems from the desirability for the surface mounting of components, from the electrical and thermal constraints imposed by high density component assembly, and by high speed circuit design. These advances make it imperative to construct PWMLBs with predictable and consistent properties in order to meet performance needs.
Of late, experience in using advanced, thermally stabilized multilayer substrates for high density assemblies shows that they are susceptible to premature failure due to the microcracking of the interconnecting conductive traces, including both the inner layer foil and the plated through hole (PTH). In some cases, fracture is initiated by a thermal stress, and the microcrack propagates during thermal cycling. In other cases, failure occurs by a tensile over-stress or by creep rupture. For either failure mechanism, the result is an open circuit condition and the loss of a completed, and usually expensive PWMLB.
Microcracking often occurs in the barrel of a PTH, at a PTH corner, or at the junction of the PTH with an interior layer. The fracture surfaces are typically quite brittle, and the studies of numerous failures has revealed little or no evidence of plastic deformation.
Fracture in a given PWMLB appears to be dependent on three factors: (1) a Z-direction thermal stress of sufficient magnitude to initiate fracture, (2) design- or process-induced stress risers, and (3) the presence of electrolytic copper with low hot strength. To date, most of the attention with regard to microcracking has been given the first two. Thus, there still remains a need to quantify the mechanical properties of copper conductors in PWMLBs with respect to strength and ductility at elevated temperatures. Since this information is obtained before the copper is used in a PWMLB, it can be used to prevent the use of a grade of electrolytic copper that is susceptible to premature thermal stress failure. In addition to eliminating failure under some conditions, this approach provides some latitude with regard to the handling of the other two factors that are mentioned as contributors to microcracking.
A printed wiring board (PWB) is the primary method now used in the electronics industry for the interconnection of circuits. A large share of the PWBs in use are produced by the fabrication of copper-clad laminates from which the copper is selectively removed by a photolithographic process to define a circuit interconnection pattern. Most of the copper used as conductor traces is supplied to laminators by a relatively small number of producers who make it in foil form by electrolytic deposition. The laminator then applies this foil to a dielectric substrate for subsequent sale to the PWB fabricator.
When the user of the laminates completes the fabrication of a multilayer printed wiring board, the circuit traces on each of the individual layers are interconnected by a plated-through-hole (PTH) process. Therefore, all of the conductors on a structure such as this are made of electrolytic copper, the surface and inner layers being done by an outside source, and the PTH deposition being done by an in-house plating operation.
It is possible to some extent to control the plating process and control contamination of the copper electroplate by means of a plating bath analysis. However, along with a capability for monitoring the condition of the plating solution, there is a need for determining and monitoring the elevated temperature mechanical properties of the resultant deposit. Control over these properties can provide control of the microcracking phenomenon.
A basic procedure for bulge testing samples of electrolytic foil has been described in T. A. Prater and H. J. Read, The Strength and Ductility of Electro-deposited Metals, Part I, Plating, Dec. 1949, and Part II, Plating, Aug. 1950. This procedure is also discussed in V. A. Lamb, C. E. Johnson, and D. R. Valentine, Physical and Mechanical Properties of Electrodeposited Copper, J. Electrochemical Soc., Oct. 1970. The Mullen rupture test of hydraulic bulge test as identified above, is widely used to determine the stress-strain properties of metal sheets and foils. However, the rubber diaphragm and hydraulic fluid used in conventional Mullen testers preclude their use with very thin sheets of electrodeposited foils. Also, current Mullen testers do not have a capability for testing at elevated temperatures.